1. Technical Field
This invention relates generally to battery charging circuits, and more specifically to battery charging circuits that apply a small current to a battery when the battery is below a predetermined threshold voltage.
2. Background Art
An increasing number of portable electronic products are available today that are powered by way of a rechargeble battery. These products include such things as cellular telephones, portable radios, pagers, personal digital assistants, MP3 players and voice recorders. Such products owe their convenient mobility to the use of rechargeable batteries. Within these batteries, various chemistries have been used to provide electrical power. Traditionally, popular chemistries included nickel-cadmium and nickel-metal-hydride. More recently, however, designers have used lithium-based cells, including lithium-ion and lithium-polymer, due to the many advantages they offer over nickel-based chemistries. These advantages primarily include low weight and overall size with a high energy density.
Rechargeable batteries generally include a protection circuit within the battery pack. Protection circuits are prolific in battery packs employing lithium-ion and lithium-polymer cells. When rechargeable cells are charged, the voltage of the cell rises. Lithium-based rechargeable cells typically have a maximum termination voltage of 4.1 or 4.2 volts. This means that if a lithium-based battery is charged beyond the termination voltagexe2x80x94known as an xe2x80x9covervoltage conditionxe2x80x9dxe2x80x94the reliability of the cell may be compromised. Most lithium battery protection circuits known in the art sense the voltage of the cell and terminate charging by opening a switch when the cell reaches the proper termination voltage.
There is another condition however, known as an xe2x80x9cundervoltage conditionxe2x80x9d, that occurs when the cell voltage drops below it""s recommended operating point. For a typical lithium-ion cell, this voltage is about 2.5 volts. When the voltage drops below this level, possibly due to over discharge, cell manufacturers suggest that rapid charging may damage the cell by inhibiting the storage capacity. To prevent performance loss, therefore, the battery must be slowly charged until it reaches the minimum operational threshold. The slow charging current, often called a xe2x80x9ctrickle currentxe2x80x9d, is on the order of a few hundred milliamps. Once the minimum threshold is reached, a full charging current, like 1 amp for example, may be applied until the maximum termination voltage is reached.
Many prior art charging systems, like that disclosed in U.S. Pat. No. 6,002,239 to Maloizel, do not include trickle-charging circuits. Consequently, when a battery with a voltage below the recommended minimum is coupled to such a charger, either the battery is charged improperly or not charged at all. There is thus a need for a low-cost charging circuit that accommodates trickle charging.